Non-volatile memories are a large part of the microelectronics infrastructure. There is a great need for devices in which information never, or only very rarely, has to be refreshed, and are fast, small, and consume little power. Such devices, and arrays made with these devices, have been known in the art for some time. For instance, one can find information on non-volatile memories in: “Nonvolatile Semiconductor Memories, Technology, Design and Applications” Edited by Chenming Hu, IEEE Press, New York, 1991.
Electrically erasable and programmable read-only memory (EEPROM) devices are the most widely spread, and useful of all the non-volatile memories. Practically all EEPROM-s are of the floating gate type, where the presence, or absence, of a charge on a floating gate alters the threshold of the device. Thus, the information is stored in the form of charge on a floating gate. An electrically programmable device of this type has to be able to change the amount of charge on the floating gate by purely electrical means. An overview of such conventional EEPROM-s can be found in: “Endurance brightens the future of Flash, fast memory as a viable mass-storage alternative,” Kurt Robinson, Electronic Component News, “Technology Horizons”, November 1988.
EEPROM devices usually use channel hot-electron injection for programming in order to achieve a fast programming speed of less than 10 μsec. In such conventional devices, during programming operation a large drain-to-source voltage is applied and a large gate-to-source voltage is also applied. Electrons flowing from source to drain gain energy from the large drain voltage and become hot electrons. The large gate voltage attracts the hot electrons, which are confined mostly near the drain region, towards the gate electrode, thus causing a gate current to flow. This gate current charges up the floating gate, causing an increase in the threshold voltage of the floating gate portion of the EEPROM device.
Although the gate voltage and the drain voltage during programming are both large during channel hot electron programming, the voltage difference (Vgate−Vdrain) is usually almost zero, or slightly negative. That is, the electric field in the gate insulator does not favor the injection of hot electrons from near the drain region into the gate insulator. Consequently, only a small fraction of the hot electrons near the drain actually contribute to the gate current, making channel hot electron programming a very inefficient process. For a typical EEPROM device, the maximum ratio of gate current to channel current is in the range of 10−11 to 10−8, depending on the details of the device design and the voltages applied. With such a low programming current efficiency, typical EEPROM device requires a channel current of about 1 mA per bit during programming in order to achieve a programming speed of less than 10 μsec. The corresponding power dissipation during programming is about 5 mW per bit, assuming a drain to source voltage of 5 volt.
With such large power dissipation during programming, conventional EEPROM devices using channel hot-electrons for programming are not suitable for low power operations, particularly to battery-powered applications, where frequent reprogramming is required. As mobile and battery-operated systems are becoming more and more prevalent, there is an urgent need for EEPROM devices that dissipate relatively little power, even during programming.